In electric power transmission and distribution, volt-ampere reactive (VAR) is a unit used to measure reactive power in an AC electric power system. Reactive power exists in an AC circuit when the current and voltage are not changing at the same time. VAR power refers to an imaginary component of the total power supplied by a power generation system, resulting from driving a reactive load such that the current and voltage are out of phase with one another. Driving an inductive load causes the current to lag the voltage, and vice versa for a capacitive load.
Thus the reactive component of power can be compensated by adding capacitance/inductance as needed. The degree of reactive power is specified by displacement power factor Cos(Φ), where Φ is the phase angle between the current and the voltage. Conventionally, capacitors are considered to generate reactive power and inductors to consume it. If a capacitor and an inductor are placed in parallel, then the currents flowing through the inductor and the capacitor tend to cancel rather than add. This is the fundamental mechanism for controlling the power factor (VAR control) in electric power transmission; capacitors (or inductors) are inserted in a circuit to partially cancel reactive power ‘consumed’ by the load.
VAR control is required where grid requirements mandate leading or lagging control over the output of any grid connected power source. New grid requirements are being imposed in regions where power generation from renewable sources, in particular solar, is becoming a larger proportion of total power generation. In particular these place requirements on VAR control for reactive power correction where differing sources supplying power to the grid could otherwise force the grid into an under-excited or over-excited mode during peak and off-peak cycles of the overall system. Thus, for example, the ability to absorb or export VLRs is required by new standards in Germany, in particular VDE4105, which will be mandatory for new products from January 2012. Thus the ability to react and correct and/or actively damp VARs is highly desirable.